1. Field of the Invention
This invention relates generally to asbestos-free asphalt compositions of the type used in roof coatings and specifically to an asphalt composition utilizing finely divided polyolefin fibers and a coupling agent to hold the fibers in suspension.
2. Description of the Prior Art
Asphalt compositions comprising asphalt cutback, asbestos fiber, and particulate fillers such as limestone are well known in the roofing industry. These compositions are used in caulking and flashing, and as roofing cement and waterproofing agents in roofing applications. Increasingly stringent government standards regulating the use of asbestos have focused attention on the need for a replacement for the asbestos component in asphalt compositions. Although various materials can be used to thicken asphalt compositions, asbestos fibers provide the strength needed in many applications.
Blending synthetic fibers with asphalt is a concept which has been known for some time. For instance, U.S. Pat. No. 3,505,260 to Gene N. Woodruff, issued Apr. 7, 1970, entitled "Asphalt-Polyolefin Fiber Blends" discloses an asbestos-free asphalt composition containing polyolefin fibers in a water based system. U.S. Pat. No. 3,634,293 to Eckhard Bonitz, issued Jan. 11, 1972, entitled "Compositions Containing Bitumen and an Olefin Polymer", teaches the use of olefin polymers with a basic substance and sulfur in bituminous compositions. Replacing asbestos with polyolefin fibers has posed problems in the past, however, because the polyolefin fibers tended to separate or settle out during storage. This problem is especially troublesome in roofing applications in which the coating is often applied with a spray gun.
Recent efforts toward asbestos-free asphalt compositions have tended to be "fiber-free" in order to avoid the problem of separation of components. See e.g. U.S. Pat. No. 4,168,179 to William F. Hesseler, issued Sept. 18, 1979, entitled "Bituminous Composition" in which high structure carbon black is dispersed in asphalt cutback. However, as has been mentioned, fibers provide added strength. Fibers provide improved bond strength between the asphalt and the underlying surface as well as improved film strength in the asphalt layer itself. For these reasons, fiber containing asphalt compositions show as much as 30 percent increase in tensile strength over asphalt compositions which are fiber-free. Also certain of the fiber-free formulations are prohibitively expensive as compared to asbestos containing compositions.